Procedures such as hyperthermia, treatment of cardiac arrhythmias and heat sensitive promoters in gene therapy require temperature change monitoring. Magnetic resonance imaging (MRI) thermometry overcomes problems associated with invasive temperature monitoring techniques such as thermocouples and fiber optics. In the current MRI thermometry the temperature variation is detected by measuring small changes in the proton resonant frequency, longitudinal relaxation time or apparent diffusion coefficient. However, these techniques have low temperature sensitivity and are influenced by the local motion and magnetic susceptibility variation. Liposome-encapsulated gadolinium chelates that have phase change characteristics have been used to monitor temperature. In particular, the temperature during the phase transition can be indicative of the local tissue temperature. However, this technique provides only a single value for the temperature, not a map of temperature distribution Furthermore, this technique requires careful design for the carrier and thus can be unreliable.
Heating organs and tissues as a treatment of cancer is well known. The first reference to link heat and the destruction of cancerous growths was as early as 3000 BC in the contents of an Egyptian papyrus and later in the writings of Hippocrates. More scientifically, the connection between disappearance of tumors and high fever, either from infections or artificially induced by bacterial toxins, have initiated a concept of thermotherapy. Reports on enhanced selective thermal sensitivity of animal tumors compared with normal tissue confirm that hyperthermia may be considered as a cytotoxic agent. There has been work conducted on all aspects of heat application to tumors, which lead to rapid development in therapeutic applicator design and to sophistication of hyperthermia equipment.
Hyperthermic temperatures increase blood circulation in tumors. The increase in temperature increases the presence of oxygen-bearing blood in tumor tissues, which is critical for increasing the effectiveness of ionizing radiation. Ionizing radiation, also referred to as radiotherapy, destroys tumor cells substantially through the formation of oxygen radicals that attack cell DNA of a tumor. Oxygen-starved cells are three-times more resistant to ionizing radiation than are normal cells. Low oxygen levels in human tumors, referred to as hypoxia, have been linked to failure in achieving local tumor control through ionizing radiation. In addition, the degree of oxygen deficiency in cancerous tumors is a key predictor of the efficacy of ionizing radiation therapy. Results from molecular biology research demonstrate that hyperthermia treatments may be used in many different tumors particularly for local tumor control. Such hyperthermia treatments resulted in an increase in the survival rate of the patients, especially when hyperthermia was combined with radiation therapy.
MRI has been used to facilitate a noninvasive technique to monitor tissue conditions. Tissue temperature change in a patient have been identified using many parameters in MRI, such as, proton density, T1 relaxation time, T2 relaxation time, diffusion coefficient, magnetization transfer and proton resonant frequency shift. These techniques, however, suffer from various limitations, such as lack of linearity, low sensitivity, dependence on tissue type, sensitivity to motion, sensitivity to susceptibility artifacts, relative temperature measurement and interrelationship with many MRI parameters.